Photoinduced charge and energy transfer in biological molecules form the basis for photosynthesis and vision. The initial absorption and electron transfer steps take place on femtosecond (fs, (10−15 s) to picosecond (10−12 s) timescales, and the active molecules are usually linear chain or cyclic ring systems with extended networks of π-orbitals, leading to electron delocalization along the entire molecule, strong optical absorptions, and facile electron transport even for large molecules. The prototypical molecules in photosynthesis are metal-substituted porphyrin compounds, and conjugated polyenes. The reactions which have been most heavily studied involve intermolecular electron or energy transfer between electron-donating and electron-accepting moieties. More generally, strongly allowed optical transitions in π-conjugated organic molecules typically involve substantial redistributions of the π-electron density in the excited-state. These intramolecular charge-transfer (CT) transitions play a dominant role in determining the strength of second- and third-order nonlinear optical effects (NLO).
Materials in which the optical properties (e.g. absorption coefficient, and index of refraction) may be dynamically altered by exposure to light have a wide variety of applications in nonlinear optics. Such applications include devices to control or alter properties of pulsed laser beams (modulators, optical switches) as well as signal and image processors using dynamic holograms. It is well-established that in solid-state blends of molecular materials consisting of light absorbers and electron acceptors, the separation of charge following photoexcitation (photoinduced CT) can lead to such changes in the optical properties (see e.g., D. McBranch, Curr. Opin. Solid-State and Mater. Sci., 3, 203 (1998)). Desirable NLO effects from photoinduced CT arise from several sources: (1) creation of new optical transitions from the excited-states of the donor and acceptor species, with associated changes in absorptivity and refractive index; (2) stabilization of the excited-state lifetime, offering additional control over the NLO response time; and (3) creation of space-charge fields which drive secondary NLO effects due to other NLO chromophores.
The creation of strong, excited-state absorption transitions with metastable lifetimes has led to large enhancements of reverse saturable-absorption nonlinearities for optical limiting of high-fluence pulses. Optical limiting performance enhanced by more than two orders of magnitude was reported for conjugated polymer/C60 blended films (see, e.g., D. McBranch et al., U.S. Pat. No. 5,741,442).
The index of refraction changes associated with the creation of new CT transitions in polymer/C60 blends were measured for steady-state laser excitation (see, e.g., K. Lee et al., Phys. Rev. B54, 10525 (1996)). Recently, ultrafast holography in these charge-transfer polymers taking advantage of both photoinduced changes in optical constants and a tunable ps response time has been demonstrated (see, e.g., E. Maniloff et al., Opt. Comm. 141, 243 (1997).
Electric-field induced, long-range charge separation in photorefractive polymers in conjunction with an ordered network of NLO chromophores, have been found to produce dynamic refractive index gratings by reorientation of dipoles with large dipole moment (field-induced birefringence) and by direct modulation of the refractive index for chromophores with high second-order molecular nonlinearity (Pockels effect). The incorporation of conjugated dye donors, C60 acceptors, hole transport agents, and optimized NLO chromophores in transparent polymer hosts, has led to polymeric photorefractive materials with diffraction efficiencies approaching unity, and response times of a few seconds (see, e.g., S. R. Marder et al, Nature 388, 845 (1997)). These polymers have been applied to optical data storage, optical correlation and pattern recognition, and self-pumped phase conjugation (see, e.g., D. McBranch, Curr. Opin. Solid-State and Mater. Sci., 3, 203 (1998)).
Dynamic holographic materials offer promise for optical processing of information with potentially high information density. However, simply comparing the maximum diffraction efficiency or the response time for different materials does not allow an adequate comparison of their relative merits, since rapid data processing requires having both a large response and a rapid recording rate. Maniloff et al. have proposed as a figure-of-merit the temporal diffraction efficiency (TDE), defined as η/τ, where η is the diffraction efficiency and τ is the time constant governing the holographic buildup (see, e.g., E. Maniloff et al., Opt. Comm. 141, 243 (1997). As an example, photorefractive polymers have large efficiencies (approaching unity), but because they respond on times≧1 s, they have TDE values≦1 s−1, for light intensities of approximately one W/cm2. Holographic materials based on photo-isomerization possess TDE values in the range of 10−1–10−6 s−1, with recording intensities of 10–50 mW/cm2. Ultrafast CT holographic materials, by contrast, show diffraction efficiencies of 2% (pump fluence 300 μJ/cm2, or average intensity 300 mW/cm2) with response times of less than 1 ps, for TDE values 10–12 orders of magnitude larger than previously reported dynamic holographic materials (see, e.g., E. Maniloff et al., Opt. Comm. 141, 243 (1997)).
The challenge of preparing macroscopic solid-state materials which utilize molecular photoinduced-charge transfer and nonlinear optical moieties has been addressed in several ways. Simply combining materials which have the desired individual characteristics has proved successful in the initial demonstrations of many NLO effects identified hereinabove. For optical limiting and holographic effects using excited-state CT transitions, disordered blended materials are sufficient. However, the observation of bulk photorefractive effects requires that the individual NLO chromophores be oriented with respect to each other. Orientation stability ranging from hours to months has been achieved in initially disordered materials by electric-field poling in host polymers which are cross-linkable, or which have elevated glass-transition temperatures Tg, in order to lock the chromophores into a metastable ordered state (see, e.g., S. R. Marder et al., Nature 388, 845 (1997)). Marder et al. have also demonstrated that dynamic reorientation could be achieved using an applied field for polymers having Tg below room temperature.
An alternative strategy for constructing solid-state materials with molecular CT and NLO components is by molecular self-assembly. Using this technique, macroscopic solids have been designed and constructed molecular layer-by-layer. Several routes to solid-state self-assembly of dipolar NLO chromophores have been investigated using covalent bonding (see, e.g., H. E. Katz et al, Science 254, 1485 (1991)). Although materials having high thermal and chemical stability have been generated using these procedures, it has proven difficult to make multilayers having arbitrary thickness. Additionally, materials, which incorporate photoinduced CT effects, have not yet been produced.
Ionic self-assembly of alternating layers of positively and negatively-charged polyelectrolytes has proven to be a versatile and simple technique for rapidly constructing multilayered organic thin films having arbitrary thickness. The surface quality and layer thickness can be extremely repeatable from layer-to-layer, and a technique for making solids for a large number of systems has been demonstrated, including transparent ionic polymers (see, e.g., G. Decher et al., Thin Solid Films 210, 831 (1992)), as well as various optically- and electrically-active species, such as phthalocyanines and porphyrins (see, e.g., T. M. Cooper et al, Langmuir 11, 2713 (1991)), conjugated polymers for thin-film, light-emitting diodes (see, e.g., J. C. Foo et al., J. Appl. Phys. 79, 7501 (1996)), and polymers with NLO chromophores as side chains (see, e.g., X. Wang et al., Macromolecular Rapid Comm. 18, 451 (1997), Y. Lvov et al., Thin Solid Film 300, 107 (1997), and K. M. Lenahan et al., Adv. Mater. 10, 853 (1998). In the latter work, it was observed that spontaneous ordering of the NLO chromophores occurs at the ionic interface, and that a high degree of net orientation is maintained for several layers. However, materials which incorporate photoinduced CT effects have not yet been produced by either covalent or ionic self-assembly methods.
Accordingly, it is an object of the present invention to provide a method for the preparation of layered supramolecular materials having individual molecular layers in which charge transfer in a controlled direction occurs.
Another object of the present invention is to provide a method for the preparation of materials having a preferred direction for charge transfer over the entire structure which produces enhanced nonlinear optical effects such as photoinduced changes in the refractive index.
Yet another object of the invention is to provide a method for the preparation of layered supramolecular materials having individual molecular layers in which energy transfer in a controlled direction occurs.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.